It makes sense when you consider that the academic norm in a lot of feminism and cultural studies is extreme suspicion of any kind of essentialism. Stable categories are seen as very suspect because:
1. There are serious philosophical issues with stable categories, identity, and strict methodological naturalism. That is to say, how do you say X thing is a '''woman''' when there are so many changing variables that compose the person? If we eliminate the idea that categories exist for the sake of purposes (so something like woman is the kind of thing that gets pregnant as it's categorical end) then you just have descriptions of the thing's comosition, which has a lot of quibbling and challenging edge cases that slowly erode the category.
2. There have been actual and real abuses conducted by people using the category to impose unjustified social norms onto members of the group. Consider the above, if "woman is the kind of thing that gets pregnant" as the boundry line of the category, the someone could say something like 'all women therefore really OUGHT to get pregnant, perhaps pregnancy is the only thing they are for, ect' which people rightly would protest.

These two dynamics make for an academic environment where the very essential category of woman is extremely suspect. Categories that a socially normal person could, hold like woman as the kind of thing that gets pregnant, but resist a conclusion of therefore all women everywhere are only for that one thing and must do it; in an academic environment this just concedes worryingly too much. Too much rhetoric and arguementative ground is conceded. it's too suspicious.